During the last decade, the number of fractures related to osteoporosis, i.e. reduced bone mass and changes in microstructure leading to an increased risk of fractures, has almost doubled. Due to the continuously increasing average life time it is estimated that by 2020 people over 60 years of age will represent 25% of Europe's population and that 40% of all women over 50 years of age will suffer from an osteoporotic fracture.
Research for suitable materials to repair or replace bone segments of the musculoskeletal system has been conducted for more than a century. Graft surgery by means of autogenous bone, i.e. bone derived from another site of the body, is one of the methods used for filling a bone cavity, replace bone lost during tumor removal, etc. Autografts are clearly osteogenic, but there is a limited supply of bone. Also, the need of a second surgical site to harvest the graft subjects the patient to additional trauma. To avoid the extra trauma, allografts, i.e. a graft of bone between individuals of the same species but of disparate genotype can be used instead of autografts. Allografts, however, demonstrate a lower osteogenic capacity and new bone formation may occur at a slower rate. This type of graft also exhibits a higher resorbtion rate, a larger immunogenic response, and less revascularisation.  Another problem with allografts is that they may transfer viruses, e.g. hepatitis and HIV virus. Therefore, careful microbiological controls are necessary before transplantation can be performed.
With the aim to reduce or eliminate the need for bone grafting, research has been made to find a suitable artificial bone mineral substitute. There are, however, substantial requirements on such materials. First of all it should be possible to use them in bone defects and they should be resorbed and/or fully integrated within the bone over time. If a tumor is removed from the bone, it should be possible to inject the material and fill the cavity in the bone. Said bone minerals substitute should also be possible to use for additional fixation of osteoporotic fractures. Additionally, it should be possible to inject the material and at the same time, if necessary, contribute in fixation of the fracture. It is not essential for the bone mineral substitute material to be strong enough to stabilize the fracture. The material should, however, be strong enough to significantly decrease the time in which an external cast or brace is necessary by aiding the stability and fracture alignment. If this is possible, the patient's mobility is not limited to the same extent as would be the case if a cast was necessary for a long period of time. This results in decreased risks for stiffness, reduced mobility, and morbidity after operation and also in a reduction of the costs for society.
Therefore, ideally, a hardened bone mineral substitute material should exhibit osteoinduction, i.e. the substitute should recruit mesenchymal cells located near the implant and from revascularisation, the cells being differentiated into bone producing cells. Furthermore, the hardened material should also exhibit osteoconduction, i.e. the substitute should act as trellis for new bone formation. 
The mechanical properties of the bone mineral substitute should be as close to cancellous, i.e. spongeous, bone as possible without being brittle but does not have to be strong enough to be possible to use for full weight bearing without added support.
The substitute should also be biocompatible, i.e. accepted by the tissues with little or untoward reaction. It should be non-allergic, non-toxic and non-carcinogenic. Furthermore, the substitute should preferably be at least partly biodegradable starting postoperative but with a certain strength for 1-6 months, in some instances totally replaced by new bone in 1-2 years.
Presently, at least the following bone mineral substitutes are used for the healing or stabilizing of bone defects and bone fractures, namely calcium sulfates, as for instance calcium sulfate hemihydrate, also known as Plaster of Paris, calcium phosphates, as for instance hydroxylapatite, and polymers, as for instance polymethyl-metacrylate (PMMA).
In WO 00/45867 a hydraulic cement composition for implantation in the human or animal body is shown, which comprises a calcium source, water and a hydrophobic liquid. The hydrophobic liquid is used in amounts between 10 and 90 wt %, preferably between 30 and 60 wt %, and is able to form an emulsion with the calcium source and water. The purpose of the hydrophobic liquid in the composition is to increase the viscosity of the composition and obtain an open macroporous calcium phoshpate matrix after hardening. Components of the composition are mixed to an emulsion of the hydrophobic liquid. However, such an emulsion can not be used if a large surface area of cement particles are to be coated with a small amount of hydrophobic liquid.
Recently, in PCT/SE99/02475, an improved injectable bone mineral substitute material for filling defects in osteoporotic bone and for additional fracture fixation in  preferably cancellous bone has been developed, which comprises calcium sulfate hemihydrate, hydroxylapatite and an accelerator. Due to the addition of an accelerator the setting time period could be controlled and considerably shortened while the injectable time was still long enough to make it possible to inject the material into e.g. a bone cavity.
These kinds of substitute materials are composed of a powder component and a liquid component which are mixed at the time of surgery, thereby initiating a setting reaction. While in a fluid or semi-fluid pre-cured form, the material is injected directly into the void in the bone or at the fracture site. During the subsequent setting reaction, the material should not reach a temperature (≧44° C.) which may cause damage to the surrounding tissue. The hardened paste provides a support by mechanically interlocking pieces of broken bone as well as conforming to the contours of the gap and supporting the cancellous bone. After curing the strength should be at least equal to that of spongeous bone.
It generally takes bone fractures, particularly non-healed fractures, many weeks and months to heal completely. During this period several physical and biochemical factors influence the natural healing process. For example, a variety of genetically driven biochemical events, particularly changes in ion transport, protein synthesis and the like are involved in the repair of fractured bones. The amount of free radicals are increased, especially in inflammatory tissue in fracture repair. The callus formation starts with mesenchymal cells which are transformed into cartilage cells and increase the stability to bone.
Also the proton concentration and superoxide radicals influence the bone formation and bone regeneration. Superoxide radicals and other highly reactive oxygen species are produced in every respiring cell as by-products of oxidative  metabolism, and have been shown to cause extensive damage to a wide variety of macromolecules and cellular components. Such an oxidative stress can arise from a surplus in the amount of activated oxygen or from a reduced amount of those molecules which are able to trap the energy of these radicals, so called scavengers. The inability of the cells to remove these free radical species will result in the destruction of biomolecules and cell structures, by means of for example lipid peroxidation, and eventually cell death. Systemically given antioxidants have in animal models been shown to improve fracture healing.
In DE 197 13 229 A1 a calcium phosphate-based bone cement is described. The injectable and hardenable bone cement paste is based on bioresorbable hydroxylapatite-like calcium phosphate containing compounds which, however, contain a cationic antibiotic as an active agent. After hardening the antibiotic is released in biologically active concentrations over a long period for the treatment and prophylaxis of osteomyelitis and ostitis, especially in connection with bone defects and fractures. This is also a well-known treatment.
During the preparation of a bone mineral substitute material it is often difficult to mix the substances in such a way that the mixture can be delivered into a patient within a reasonable period of time during surgery in an operation room. For example, when the powder component of a calcium sulphate or a calcium phosphate based cement is mixed with water, this takes place in a container, from which the mixture is delivered to the treatment site via a nozzle, the material being injected under pressure. During the injection the nozzle may become clogged. The situation is aggravated if for example a fracture is to be treated through a small hole in the trochanter region and the cement has to be injected 10 cm from the injection site, a pressure being built up. Thus, the bone mineral substitute  material must be prepared in a way so that it can be easily injected and the mixing of the cement has to be made rapidly, reproducibly and with a sufficient homogeneity. Such a ceramic material should harden within 6-12 min, preferably between 5-10 min, and the viscosity of the material should be adapted to be easily injected within 5 min. In this connection it is important to prevent crack formations or defects in the hardened cement, which may be caused by insufficient packaging of the bone substitute material after injection.
The viscosity of the material should be adapted in order to be easy to inject into the bone for 1-5 minutes after start of mixing procedure. In this connection problems are often obtained when the bone cement material is injected if not handled with extreme caution. This is especially true if the cement has to be injected through a long nozzle with a small diameter. Thus, it is also important to eliminate the drawback of high viscosity at delivery by improving the rheology of the bone mineral substitute material.
There is also a demand for a bone mineral substitute material which prevents negative effects during the bone regeneration process, such as minimize the risk of infections and other complications, and improves and accelerates the tissue and bone healing at the treatment site.
Furthermore, when joint implants, e.g. hip and knee joint implants, are fixated in the bone by means of what is called a cementless fixation, it is very important that the shape of the bone cavity, into which the implant is to be placed, exactly matches the shape of the implant. In practice, the bone cavity preparation always gives rise to a mismatch to a greater or less extent between the bone cavity and the implant. This mismatch results in a reduction in stability and a decreased probability of a successful bone ingrowth and ongrowth onto the implant  surface. The degree of bone ingrowth/ongrowth is in turn extremely critical for the long term fixation and thereby the survival of the implant.